I have a little predicament. I bought some slick tires for my mountain bike that I am about to use to bike across Wisconsin on Friday. I coulda swore that they were rated to 100 psi when I bought them, but it turns out they are only 60.
My front one seemed fine, but with the extra weight in my panniers and my weight on the back, the tire seemed about half inflated at 60 psi. Should I try going to 80 or should I just buy a tire that is rated to go higher?

I would normally buy a new one. I guess it is possible that it is rated at sixty just because of some design issue, like to make it more comfortable to ride. Often over inflating causes the tire to expand right off the rim. So the remaining question would be if you pump it up and it doesn't get way fatter, or pop free of the rim, does that mean it is safe to ride it at the higher inflation. I don't know the answer, but you could try overinflating it since on the occasions when I have done it, it hasn't been damaged, it just didn't work.

I regularly inflate higher than the rated pressure. The jump from 60-80 is a big one though and Friday doesn't give you much time to see how they do at that pressure. 10-15% over I wouldn't be worried at all. If it was me I would pump them up now and ride until then at 80 psi and decide at the last minute depending on how they did. It could bite you on the ass though.

If that tire at 60 lbs looks half flat, then it's not a tire rated / designed for loaded touring. I would replace it. How much weight do you have up-front? Probably the front will be fine unless you have heavy front panniers as well.

On loaded touring bike, I like the rear tire in the high end of the recommended range and the front at the low end of the range. When un-loaded, they are both on the low end, nicer ride.

There have been several good articles written about tire pressure and rolling resistance, here is one from Sheldon Brown.

"Rolling Resistance
"Rolling resistance" is the mechanical friction generated as the tire rolls. As a segment of the tire tread rolls into contact with the road, it deforms from its normal curved shape into a flat shape against the road, then back to the curve as the tire rolls onward. The deformation of the rubber in this process is what causes the friction.
There are two ways to reduce this friction, each subject to trade-offs:

The thinner and softer the rubber/fabric of the tire are, the more flexible they become.
The trade-off with this is that the thinner the tire gets, the more fragile it is, and the sooner it will wear out.

The higher the air pressure, the less the tire will deflect.
The trade-off with this is that if you pump the tire up too hard, you lose the benefits of pneumatic tires: the ride becomes excessively harsh, and traction will be reduced. In addition, extremely high pressures require a stronger (heavier) fabric and stronger (heavier) rim flanges.

Rolling resistance does decrease theoretically with any increase in pressure, but with modern, high-quality tires the rolling resistance at correct inflation pressure is already so low that the infinitesimal reductions gained are more than outweighed by the trade-offs."

I don't have front panniers, so all the weight is in the back. Its not much weight though, probably around 20-25 pounds, I'll be weighing it out this evening just out of curiosity. I think I will buy a new tire for the back though, one that I can pump to at least 80 psi.

Well I ended up buying a tire that was good to 100 psi, and it was great. I had no flats on the entire ride. I was planning on doing 120 both days, but I did 160 and 80 instead. Wish I could change the title of this thread to "Seat Problem" or "Padded Shorts Problem" after all those miles, I definitely don't want to sit on a bike seat anytime soon.